1. Field of the Invention
The present invention relates to an optical pickup actuator, and more particularly, to a stable optical pickup actuator which exhibits increased gain margin and reduced vibration by employing a damping member at a position and reducing the size of a second resonant peak.
2. Description of the Related Art
Generally, optical recording and/or reproducing apparatuses for recording and/or reproducing information on and/or from an optical recording medium, that is, a disc, include an optical pickup actuator which radiates light onto a recording surface of the disc and receives light reflected from the disc while moving along a radial direction of the disc, so that information is recorded or reproduced.
Referring to FIGS. 1 and 2, in a conventional optical pickup actuator, a holder 13 is formed at one side of a base 10, and a bobbin 17 on which an objective lens 15 is placed is mounted in a central portion of the base 10. Throughholes 18 are formed at both sides of the objective lens 15 on the bobbin 17, and first magnets 20 are inserted into the through-holes 18. Second magnets 23 are installed at both sides of the bobbin 17. The first and second magnets 20 and 23 are attached to first and second yokes 25 and 27, respectively, formed on the base 10. Meanwhile, third magnets 30 are provided at both sides of the base 10 in a tracking direction T. The third magnets 30 are attached to third yokes 33 formed on the base 10.
The bobbin 17 is suspended and supported movably by suspension wires 35 that are fixed to the holder 13 at one end.
Turning now to FIG. 2, magnetic driving unit is provided for driving the bobbin 17 in a focusing direction F, the tracking direction T, and a tilting direction t. The magnetic driving unit includes tracking coils 40 wound around an inside wall of the through-hole 18, focusing coils 43 wound outside the through-hole 18, tilting coils 45 wound around both sides of the bobbin 17 in the tracking direction T, and the first to third magnets 20, 23, and 30, respectively.
When power supply is applied to the tracking coils 40, the focusing coil 43, and the tilting coils 45, the bobbin 17 operates in the focusing direction F, the tracking direction T, or the tilting direction t by an interaction between the coils 40, 43, and 45 and the first to third magnets 20, 23, and 30, respectively, so that focusing, tracking, and tilting operations of the objective lens 15 are performed.
Since the bobbin 17 is suspended by the suspension wires 35, the optical pickup actuator has a vibration characteristic. In order to measure the vibration characteristic, the optical pickup actuator constitutes an open loop with respect to a phase and a gain with respect to a frequency. FIG. 3A shows a form of a general open loop. In FIG. 3A, a frequency “a” corresponding to 0 dB indicates a 0 dB cutoff frequency, and a point p indicates a second resonant peak. A difference between a gain at the 0 dB cutoff frequency and a gain at the second resonant peak indicates a gain margin GM.
However, as the speed of recording media increases, an operation frequency increases correspondingly in focusing and tracking operations of an optical pickup, thereby generating problems of deflection and eccentricity of the recording medium. Thus, in order to employ a high-speed optical recording apparatus, problems caused by disturbances and increased acceleration of deflected and eccentric discs must be solved. In order to solve such problems, the 0 dB cutoff frequency “a” in the open loop of FIG. 3 must be increased. In order to increase the 0 dB cutoff frequency, a gain and a phase in an RF chip, a drive IC chip, and a digital equalizer (DEQ) must be changed. However, a second resonance occurs inevitably at a frequency of about 20 kHz or more due to a physical structure of the optical pickup actuator, and a gain increases greatly in a second resonance frequency area within which the second resonance occurs. Further, a gain at the second resonant peak may exceed 0 dB. In this case, a gain margin is 0.
In a case where a gain margin is reduced, it is more likely that an optical recording apparatus oscillates when disturbances having the second resonance frequency and the second resonance frequency/n (n is natural number) is input to the optical recording apparatus. Because of this oscillation possibility, the gain margin obtained from the second resonant peak in driving the high-speed optical recording medium is an important design condition in designing a controller of the optical recording apparatus.
In order to ensure a proper gain margin, there are a first method for increasing the second resonance frequency and a second method for reducing a size of the second resonant peak. As shown in FIG. 3B, attempts to ensure a proper gain margin using the first method have been made. The size of the second resonant peak decreases as the second resonance frequency increases. Thus, as shown in FIG. 3B, the gain margin is increased from GM1 to GM2 (GM1<GM2) by increasing the second resonance frequency from c to d.
However, it is difficult to significantly increase the second resonance frequency without changing the entire structure of the conventional optical pickup actuator.
A material having high rigidity is frequently used in order to increase the second resonance frequency. When the second resonance frequency is increased using the material having high rigidity, as shown in FIG. 3C, the second resonance frequency increases from e to f, but the size of the second resonant peak also increases. Consequently, the gain margin is reduced from GM3 to GM4 (GM3>GM4).
FIGS. 4A and 4B are a design view of an EQ (equalizer) with respect to the conventional optical pickup actuator. According to the design view of the equalizer, gain is increased automatically in order to overcome eccentric acceleration in recording and/or reproducing of a high-speed recording medium so that a 0 dB cutoff frequency increases. Here, a problem of the actuator becoming unstable due to the increase of the 0 dB cutoff frequency must be solved. FIG. 4B shows gain and phase after coefficients of the controller are designed to perform phase compensation for ensuring the stability of the actuator in the frequency range of 20-30 kHz where a second resonance frequency of the actuator is. FIG. 4B shows a second resonance frequency band of 20-30 kHz, and it can be seen that gain increases about a factor of 10 due to the design of the coefficients of the controller for phase compensation. This indicates that the gain margin is reduced inevitably in the second resonance frequency band.
Thus, the second method for reducing the size of the second resonant peak in the second resonance frequency band needs to be newly studied, and compared with the first method for raising the second resonance frequency in order to ensure a proper gain margin.
FIG. 5A is a graph illustrating the vibration characteristic of a 2× CD, and FIG. 5B is a graph illustrating the vibration characteristic of a 20× CD. Here, portions A and B indicate a second resonant peak. As the speed of the recording medium increases, the 0 dB cutoff frequency increases and the size of the second resonant peak increases greatly. In other words, as the speed of the recording medium increases, the gain margin is reduced greatly. Thus, it becomes increasingly essential to ensure a proper gain margin as the speed of recording media increases.